1. Field of the Invention
The present invention relates to a spoke wheel for use in a motorcycle and the like. More specifically, the invention relates to a spoke wheel suitably used for a tubeless tire spoke wheel having a rim with a flange that is formed to project in an inner diameter direction. The spoke wheel is assembled by anchoring stopper heads of wire spokes to the flange of the rim and by fastening the spokes by nipples at the hub.
2. Description of the Related Art
A motorcycle wheel is composed of a hub attached to an axle, a rim on which a tire mounts and spokes connecting the hub with the rim in general. While there are a cast wheel in which the rim, spokes and hub are integrally molded by casting and a spoke wheel (or more accurately a wired spoke wheel) in which the rim is connected with the hub by a large number of wired spokes, the present invention relates to the spoke wheel. Because the spoke wheel can absorb impacts by deforming its rim and spokes, in addition to that its weight can be lightened, the spoke wheel is adopted in many motorcycles including cruiser-type and off-road-type motorcycles.
Japanese Patent No. 3237721 discloses a spoke wheel in which an annular flange is formed on an inner periphery of the rim, heads of spokes are stopped and anchored to the flange, and male screw portions of the spokes are fastened to ear portions formed on the hub by nipples. Because the spoke wheel has the rim formed of an aluminum mold material and requires no through hole for anchoring the spokes in a well (drop) portion and others thereof, it becomes easy to make the spoke wheel tubeless and to thin the rim including the well portion, as compared to one which is made tubeless by closing spoke anchoring holes perforated through the well portion by rubber caps. Thus, the spoke wheel is preferable also for lightening a weight of the wheel.
Hitherto, as shown in FIG. 7, the rim 1 with the flange as described above is provided with spoke receiving holes 3 perforated so as to be orthogonal to a center (rotational) plane including a center line O-O of the rim 1. Each spoke receiving hole 3 is provided with tapered surfaces 3a and 3b that open to flange surfaces 2a and 2b on both sides of the spoke receiving hole 3. Each spoke 5 is inserted through the spoke receiving hole 3 from a male screw portion thereof (not shown), and a head 6 thereof is seated and anchored to one tapered surface 3a. Each spoke 5 is bent at a curved portion C1 from the head 6 through a relatively short straight portion 5a or a portion bent with a large radius and extends straightly from the curved portion C1 as a body straight portion 5b. The male screw portion 13d located at an edge of the body straight portion 5b is fastened to the hub by a nipple (not shown) to apply a predetermined tensile force F to the spoke 5.
Therefore, based on the tensile force F1 acting on the head straight portion 5a, the head 6 of the spoke 5 contacts with the flange 2 at a region D close to a rim body 1a and a region E distant from the rim body 1a in an edge part (small diameter part), i.e., a boundary part, of one tapered surface 3a of the spoke receiving hole 3. The spoke 5 also contacts with the flange 2 at a region G distant from the rim body 1a in a small diameter part, i.e., a boundary part, of the other tapered surface 3b of the spoke receiving hole 3 with the curved portion C1.
A compressive force acts on a spoke set of the spoke wheel at part contacting with the ground. Thereby, the spoke wheel receives a force that deforms the rim flat, and tensile forces act on right and left (in a front-back direction of the wheel) spoke sets. Specifically, very large compressive and tensile forces act repetitively on the spokes in the cruiser-type motorcycle whose weight is heavy or in the off-road-type motorcycle to which a large impact load acts after jumping and others. The contact of the spoke 5 with the flange 2 at the contact portion G does not act as a part for locking the spoke 5 and a tensile force F2 from the body straight portion 5b acts directly on the head straight portion 5a as a tensile force F1 and acts on the contact portions D and E of the head 6. Specifically, as shown in FIG. 7, the tensile force F2 from the body straight portion 5b acts as a large force pressing the contact portion D on the side closer to the rim body 1a as a moment on the fulcrum of the contact portion G. Thereby, a large concentration of load occurs in the base portion of the flange 2, causing a fatigue breakdown of the rim 1 and the spoke 5.
Strength of the flange is considered to be dependent on stress received from the spoke receiving hole 3. Hence, the thicker the thickness h of the flange on the inner diameter side of the spoke receiving hole 3, the higher the strength is. Therefore, the spoke receiving hole 3 is made on the side closer to the rim body 1a of the flange 2. In this case, the tapered surfaces 3a and 3b are formed so as to bite from starting-end portions e of curved surfaces 2a1 and 2b1 from the rim body 1a of the flange 2. Such tapered surfaces 3a and 3b act as a notch effect of the base portions of the flange 2 to the large concentrated load from the contact portion D and are presumed to be a large because of the drop in the fatigue strength.
While a predetermined angle is required for the bending angle of the curved portion C1 of the spoke 5 in order to assure a plane angle (see α in FIG. 1) and an elevation angle (see β in FIG. 2) in tensioning the spokes, the bending angle of the curved portion becomes relatively small (formed in an acute angle direction) because the spoke receiving hole 3 is formed perpendicularly to the center plane O-O of the rim 1 in the spoke wheel described above. The more acute the curved portion of the spoke, the greater the local deformation of the curved portion becomes when a load greater than a certain degree is applied, so that the spoke is liable to be elongated and drops the tension thereof (the spoke is loosened). Still more, the compressive force (a force in a direction of thrusting up the spoke) repetitively acting on the spoke 5 is not caught at the contact portion G and is propagated to the contact portion D (E) of the head 6. The spoke 5 becomes liable to be loosened when a compressive force exceeding the tensile force applied to the spoke acts on the spoke wheel and if the spoke 5 is loosened due to the elongation of the spoke described above and to the compressive force directly propagated to the contact portion D, a wheel balance of the spoke wheel keeping a circular shape by balanced tensile forces of the respective spokes collapses due to the loosened partial spokes. Then, the spoke wheel ends up causing vertical and horizontal vibrations and losing normal functions as a spoke wheel.
While the prior art spoke wheel is arranged to disperse the concentrated load otherwise applied from the spokes to the flange and to prevent the spokes from being loosened by increasing a diameter of the spokes or by increasing a number spoke sets, this measure causes an increase in weight of the spoke wheel, increases unspring weight and wheel inertia (effective inertia) and leads to a drop of running stability.
Still more, the hub described in Japanese Patent No. 3237721 is arranged so as to connect right and left axle supporting sections by a cylindrical section having the same diameter with the supporting sections, and ear portions for fastening spokes are formed respectively radially on an outer diameter side of the right and left axle supporting portions. That is, this hub requires the separate spoke connecting sections respectively composed of rise portions extending in the radial direction and the flanges extending from the rise portions in the axial direction. This arrangement is disadvantageous in terms of strength and causes an increase in weight. This is one cause of the increase in weight of the spoke wheel and leads to a drop of performances as a spoke wheel as described above.